1. Field of the Invention
The present invention relates to an illumination intensity correcting circuit for correcting the intensity of a video signal or three primary color signals.
2. Description of the Related Art
FIG. 1 is a schematic block diagram of a computer display apparatus of the related art. This computer display apparatus 100 includes a signal amplifier circuit 20, a cathode ray tube ( hereinafter referred to as a CRT) 30, and a deflection circuit 40. The signal amplifier circuit 20 includes first to third amplifier circuits 21 to 23. The CRT 30 includes an electron gun 34. The first amplifier circuit 21 receives as an input a red primary color signal SR to amplify the same and supplies the resultant signal SR2 to a cathode 31 of the electron gun 34. The second amplifier circuit 22 receives as an input a green primary color signal SG to amplify the same and supplies the resultant signal SG2 to a cathode 32 of the electron gun 34. The third amplifier circuit 23 receives as an input a blue primary color signal SB to amplify the same and supplies the resultant signal SB2 to a cathode 33 of the electron gun 34.
The electron gun 34 generates electron beams BR, BG, and BB according to the output signals SR2, SG2, and SB2 correspondingly and emits the electron beams BR, BG, and BB to a fluorescent screen 35. This fluorescent screen 35 is made of a fluorescent material that is coated on the inside of the front glass of the CRT 30. The deflection circuit 40 receives as inputs horizontal and vertical synchronizing signals SH and SV and generates a magnetic force by a deflecting coil (yoke) in accordance with the signals SH and SV to change magnetic field force in the CRT 30 so that horizontal and vertical scanning by the electron beams from the electron gun 34 are carried out in the CRT 30. The three primary color signals SR, SG, and SB are analog output signals for example from digital-to-analog converters which are supplied with digital signals respectively from frame buffers used for the three primary color signals or from color look-up tables (CLUTs).
Known in the art is a "curve fitting circuit" for compressing the amplitude of an input signal by approximating the signal characteristic by a series of breakpoints connected by straight lines. For example, the curve fitting circuit is formed by a parallel-connected circuit which consists of a reference resistor (first resistor) and a serial-connected circuit that includes a diode, a second resistor, and a constant-voltage source. This constant-voltage source generates a reference voltage. If the terminal voltage of the reference resistor is smaller than the reference voltage, the diode is supplied with a reverse voltage and turned OFF, whereby the parallel-connected circuit is given a first electric resistance the same as that of the reference resistor. If the terminal voltage of the reference resistor is larger than the reference voltage, the diode is supplied with a forward voltage and turned ON, whereby the parallel-connected circuit is given a second electric resistance of the reference resistor and the second resistor connected in parallel. If the parallel-connected circuit is used as a load resistor, the electric resistance of the load resistor is changed before and after the reference voltage, so the parallel-connected circuit can form a curve fitting circuit.
However this curve fitting circuit is structured so that the diode is supplied with a reverse voltage and forms a depletion layer capacitance when the reverse voltage is supplied. Since the resistance of the parallel-connected circuit differs for a low frequency component and a high frequency component of the same input signal due to the capacitance of the depletion layer, this curve fitting circuit is not well suited for high speed operation.
Moreover the diode resistance changes according to the applied voltage of the reference resistor due to the non-linear characteristic of the diode, so this curve fitting circuit may suffer from distortion near each breakpoint in the signal characteristic.
A television receiver is sometimes provided with an illumination intensity correcting circuit for adjusting the relationship between the three primary color signals or video signal and the brightness of the image. For example, use is sometimes made of a gamma correcting circuit. As the curve of the gamma characteristic, use may be made of a convex secondary degree curve. Because the gamma correcting circuit of the related art forms a secondary degree curve by using capacitor elements, it is difficult to perform gamma-correction on the high frequency component of a video signal or three primary color signals.